Generally, RFID systems are expected to become ubiquitous so that it would be beneficial to integrate RFID readers into various devices such as mobile phones. However, an RFID reader consumes energy and shortens the battery charging cycle in battery operated devices. Most of the energy consumption caused by an RFID reader is caused by the need to perform repeated searching to identify proximate tags. Before accessing an RFID communication unit such as an RFID tag, a scan or interrogation procedure is performed. During the interrogation, proximate tags are searched with a reader by radiating electromagnetic fields at a suitable frequency band such that a proximate tag is able to draw sufficient power from the radiation to, for example, alter its back scattering so as to cause a response signal that is recognisable by the reader. Without interrogation, the tags remain silent and invisible to the reader. Hence, in order to provide a user experience of automated access resulting from bringing a tag and reader into a short mutual distance, frequent interrogation is required. For instance, for an access control system in which an RFID tag operates as a remote key that unlocks a door, the interrogation should be repeated a number of times each second or the system appears unreliable to the user. As each interrogation consumes a given amount of energy, the power draw of the interrogations is directly proportional to the frequency at which the interrogations are performed.
There may also be a need to scan tags of more than one RFID system as different RFID systems are employed for different purposes. Among the most prominent RFID systems there are Near Field Communication (NFC) and EPCGlobal (EPC refers to Electronic Product Code) systems which use different frequencies or frequency bands and which also have regionally differing frequency variants according to national or regional frequency allocations. In order to scan different types of tags, correspondingly multiplied number of scans is required, with consequently increased battery draining.
Another problem involved with integration and use of various RFID systems in mobile phones relates to the co-operation of radio transceivers on Ultra High Frequency (UHF) band. For example, the RFID reader transceivers operating according to EPCGlobal standard and GSM transceiver (900/850 MHz) operate on frequency bands which are very close to each other. It may be very difficult to provide sufficient isolation between such two systems especially when using a relatively small device such as a mobile phone to host an RFID reader. As result, the use of such different system communicating substantially within common frequency band must be interleaved in time. The time interleaving is particularly challenging in connection with scanning for proximate tags when GSM time slots or any other communication on either uplink or downlink cause interruptions to the periods over which RFID scanning could be performed.
It is also challenging to integrate RFID readers into handsets to achieve reasonable isolation between transmitting and receiving paths of RFID TRx since transmission and reception should be active simultaneously and the transmission power easily leaks to the receiving path. This problem effectively restricts the operation range and/or data rate of RFID communications. One solution to this is to use one frequency band UHF 900 MHz for powering (900 MHz transmission only in reader) and to perform communication between the tag and reader on entirely different frequency band such as 3-5 GHz UWB band with time-multiplexed transmission and reception.
WO2006070237 is another application assigned to the patentee of this patent application. This publication describes an RFID system based on Impulse UWB (I-UWB) radios. In the publication, tags obtain operational power from signals transmitted by remote wireless communication devices. Tags may be partly powered by Bluetooth signals. However, even when Bluetooth signals are used to aid powering of tags, normal RFID interrogation signals are still needed and thus scanning different types of RFID tags has to be performed one by one for each type. Hence, numerous scans are needed.
In WO2006055431 an asymmetric communication method is described in which downlink communication and wireless power transmission from reader device to tag device are performed by using a narrow-band signal. The uplink communication from tag device to reader device is carried out over Time Domain Carrierless Impulse Radio (TDCIR). The publication also discloses separate embodiments in which a system communicates over a bi-directional Ultra Wide Band (UWB) link when the wireless power transmission is implemented by using a narrow-band signal on HF, UHF or any other frequency band or combination of the signals. While the method of the publication suggests that the system may be compatible with legacy RFID systems, separate interrogation for legacy RFID and TDCIR or UWB systems would be needed. It is also mentioned in third embodiment of this publication that other narrow-band signals than the one specified for interrogation can be used for powering of tags. However, a dedicated interrogation signal is always used by the reader to carry a clock signal which may be recovered in the tag from the incident wave.
It is an object of the invention to avoid or at least mitigate problems related to prior art and/or to provide a new technical solution.